1. Field of Invention
The present invention relates to a process for preparing cellulose ether products which is characterized in that superabsorbent polymers (SAPs) are dried and milled, or mill-dried, conjointly with moist cellulose ether and also to the cellulose ether products resulting from this process.
2. Description of Related Art
Cellulose ethers have outstanding properties and therefore are widely used, for example as thickeners, adhesives, binders, dispersants, water retainers, protective colloids, stabilizers and also as suspending, emulsifying and filming agents.
It is common knowledge that cellulose ethers are prepared by alkaline destructurization of cellulose and subsequent etherification with etherifying reagents such as methyl chloride, ethylene oxide and propylene oxide. However, for the as-synthesized crude cellulose derivative to be useful in industrial applications it must first be washed, densified, milled and dried.
Its viscosity in aqueous solution is a pivotal factor in deciding the possible industrial uses of a cellulose ether. Its viscosity in an aqueous solution is chiefly determined by the chain length of its cellulose molecule and accordingly is dependent on the raw material used.
There continues to be a demand for production processes which for any given raw material lead to an increase in the viscosity of the product.
It is known for example from EP 0 384 046 that the type of mill chosen can have an effect on the viscosity of the cellulose ether obtained.
It is further well known that combining cellulose ethers with other additives or fillers makes it possible to achieve specific properties for industrial formulations.
U.S. Pat. No. 4,487,864 discloses a composition comprising a polyacrylamide-based SAP and a cellulose ether. It is said to achieve improved water retention in various building construction applications such as tile adhesives or plasters than is achieved with a cellulose ether alone. The compositions used are blends of powders of the individual components. These contain from 0.05% to 50% of the SAP.
A similar powder blend of cellulose ethers and SAP and also its use in wallpaper adhesives is described in DE 39 29 400.
EP 0327351 is to a composition wherein the SAPs are based on polyacrylic acid. The ratio of cellulose ether to SAP is in the range from 3:1 to 1:3 parts by weight. The preferred ratio is 1:1. The compositions used are blends of powders of the individual components.
The paper “Highly effective thickening systems with cellulose ether combinations” Cellulosics: Chem., Biochem Mater. Aspects (1993), 325-330 describes synergistic effects with regard to the viscosity in water on concurrent use of MHEC and SAP in a ratio of 1:1 parts by weight.
However, the processes described above are unable to provide products having a viscosity required for some applications. In addition, the fineness of the products prepared by the processes described above is in need of improvement.